Want to reduce obesity? Legalize medical marijuana, researchers say.

Elan Nelson, a spokesperson for a marijuana retail and grow facility, answers questions amid a forest of marijuana under cultivation in April. (AP Photo/David Zalubowski)

Want to take a bite out of the American obesity epidemic? Legalize medical marijuana.

That's the counterintuitive finding of a new study in the journal Health Economics. Researchers from San Diego State University and Cornell University found that at the state level, passing a medical marijuana law (MML) "is associated with a 2 percent to 6 percent decline in the probability of obesity." Over the longer term, this effect could be even larger. Tally it all up and according to the study's authors, "weestimateaback-of-the-envelopeper-person reductioninMML-inducedobesityrelatedmedicalcostsof$58to$115peryear."

Taken at face value this doesn't exactly make sense: Marijuana is a well-known appetite stimulant. Smoke a joint, and before you know it you're reaching for the Funyuns. Or, as the study's authors put it in reseacher-ese: "randomized control trials provide evidence that marijuana use leads to increased appetite and caloric intake."

But the researchers wanted to know how increased medical marijuana availability could affect a variety of health outcomes at the societal level. So they analyzed over 20 years of data from the federal Behavioral Risk Factor Surveillance Survey (BRFSS), including over 5 million individual survey responses. And during the time period they studied (1990-2012), a number of states implemented medical marijuana laws, while many others didn't. Voila: natural experiment.

They found that passage of medical marijuana laws was associated with declines in obesity and overall BMI, controlling for social and economic factors, policy differences and food prices. As for the "why," the researchers found that a number of factors may be at play.

For older adults, they found that medical marijuana laws "areassociatedwithanincreaseinphysicalwellnessandfrequentexercise." The possible reason? People are often prescribed medical marijuana for chronic pain, one of the conditions it's most affective at treating. Older patients are more likely to experience chronic pain. So if medical marijuana reduces pain, it may be allowing those patients to be more active -- and hence, burn more calories.

But for younger adults, age 18 to 24, the study found a different dynamic at play. "Our findings show that the enactment of MMLs is associated with a 3.1 percent reduction in the probability of alcohol consumption and a 4.8 percent reduction in the probability of binge drinking" among this younger group, the researchers found. They posit that medical marijuana availability may lead some younger adults to "substitute away from highly caloric alcoholic beverages toward a lower-calorie marijuana 'high,' resulting in lower body weight and likelihood of obesity."

This so-called substitution effect is often cited in arguments for legalizing marijuana: If you legalize weed, some people will opt for pot over alcohol. Alcohol consumption will fall as a result. And since researchers agree that marijuanaisfarless harmful than alcohol, the net effect of such a change for individuals and society would be positive.

Some researchers caution against reading too much into the results of this one study. Rosalie Pacula, director of the BING Center for Health Economics at the RAND Corp., says that the nationally representative BRFSS data is not necessarily the best for sussing out state-level effects. Beyond that, in a number of the states in this study, medical marijuana laws are still very new, so the data on the impact of those laws are relatively sparse.

"These data aren’t going to provide us with the definitive answer because of the issues with the data and time period being evaluated," Pacula said in an email. "Thus, I believe that these findings should be interpreted very cautiously – as they could plausibly be reversed in the future, when other analyses are done using data sets that contain either (a) state representative populations over time, or (b) longer time periods for later policies."

In the end, after all, we're looking at just one study. As always with these things, it's far from the last word on the matter.